1) Field of the Invention
The present invention relates to a control magnetic bearing which is used in a turbo-molecular pump and rotates a rotator without the rotator and the magnetic bearing making any physical contact.
2) Description of Related Art
A control magnetic bearing of this type comprises a mechanical main unit and a controller connected to the main unit by way of a cable.
The mechanical main unit comprises a plurality of displacement sensors for detecting axial and radial displacements of the rotator; plural sets of magnetic bearings for supporting the rotator in both the axial and the radial directions using attractive magnetic forces generated by a pair of electromagnets that constitute each of the magnetic bearings; an electric motor for rotating the rotator; and a protective bearing for limiting a movable range of the rotator. The protective bearing limits the movable range of the rotator in both the axial and radial directions and mechanically supports the rotator when the magnetic bearings lose magnetic forces to support the rotator.
The controller controls an excitation current supplied to the electromagnets, in response to signals output form the displacement sensors and through the use of predetermined control parameters.
The mechanical main unit of such magnetic bearing differs in type according to the application of the magnetic bearing. When the mechanical main unit is changed to another type, control parameters used by the controller must also be changed. The type of the mechanical main unit differs according to the characteristics of the rotator (such as the weight, characteristics frequency, and position of the center gravity of the rotator), the type of electromagnet, and the positional relationships among the rotator, the electromagnets, and the displacement sensors. For this reason, traditionally there have been prepared a plurality of different types of mechanical main units and a plurality of different types of controllers employing different control parameters. The mechanical main units and the controllers are used in combination according to the application.
However, such a traditional practice is uneconomical in that it requires plural types of controllers to conform with the types of the mechanical main unit. The erroneous combination of the mechanical main unit and the controller may make unstable the positional control of the rotator by the magnetic bearings control since parameters set in the controller are not suitable for use with the magnetic bearing. To prevent such unstable positional control of the rotator, desirably the controller automatically identifies the type of the mechanical main unit when it is connected to the mechanical main unit, and checks whether or not the controller is suitable for use with the thus-connected mechanical main unit. The magnetic bearings primarily employ an analog PID control method as a control method for the controllers. Further, the controllers have only the function of outputting a signal to the electromagnets in response to the signals received from the displacement sensors. Thus, the controllers have been unable to identify the type of the mechanical main unit.